A cordless telephone system typically includes a portable cordless handset and a cordless base station connected to a telephone company phone system by telephone landlines. The cordless base station has an assigned landline telephone number that allows the user to place and receive calls using the cordless portable handset within a limited range of the cordless base station, such as in a home. However, due to their limited range, the cordless portable handset provides the user with relatively local radiotelephone communication.
Radiotelephone communication outside the range of the cordless telephone system may also be provided to the user via a cellular telephone system. A cellular telephone system typically includes cellular subscriber units (mobile or portable) and cellular base stations connected to the landline telephone system via one or more cellular switching networks. Each cellular subscriber unit has an assigned cellular telephone number that allows the user to place and receive calls within a widespread range of the cellular base stations, such as throughout a metropolitan area. However, the cost of using the cellular telephone service is much greater than the cordless telephone service.
A problem exists for the user that frequently relocates between the cordless and cellular telephone systems. An incoming call routed to the system where the user is not located may be missed. In the prior art, landline and cellular telephone companies have provided a solution to this problem with features known as No Answer Transfer or Call Forwarding or Three Way Calling. No Answer Transfer enables the user to program the system to route an incoming call from the cellular telephone system to the cordless telephone system or visa versa when the user's telephone that was called is not turned on, not answered, or out of range of the base station. Thus, the user may receive an incoming call placed to either the cordless or cellular telephone system.
Some problems exist with the No Answer Transfer feature. The user must manually program the system each time there is a need to activate or deactivate the No Answer Transfer feature. Manually programming the system is a cumbersome task for the user and forgetfulness of the user may result in missed or improperly routed incoming calls. The user must also purchase and operate unique radiotelephone equipment for both the cordless and cellular telephone systems resulting in increased cost and inconvenience to the user.
Additionally, a portable radiotelephone which transmits and receives calls in both a cellular and a cordless system should have the capability to select the clearest available channel for use. An automatic channel selection should minimize interference and eliminate the need for conventional frequency planning.
Accordingly, there is a need for a radiotelephone system that enables a user to receive incoming calls via both a cordless and cellular telephone system and automatically selects the optimum channel.